1. Field of the Invention
The invention relates generally to blowout preventers used in the oil and gas industry. Specifically, the invention relates to a ball valve blowout preventer with two biasing seats, allowing the valve ball to float.
2. Background Art
Well control is an important aspect of oil and gas exploration. When drilling a well, for example, in oil and gas exploration applications, safety devices must be put in place to prevent injury to personnel and damage to equipment resulting from unexpected events associated with the drilling activities.
Drilling wells in oil and gas exploration involves penetrating a variety of subsurface geologic structures, or “layers.” Occasionally, a wellbore will penetrate a layer having a formation pressure substantially higher than the pressure maintained in the wellbore. When this occurs, the well is said to have “taken a kick.” The pressure increase associated with the kick is generally produced by an influx of formation fluids (which may be a liquid, a gas, or a combination thereof) into the wellbore. The relatively high pressure kick tends to propagate from a point of entry in the wellbore uphole (from a high pressure region to a low pressure region). If the kick is allowed to reach the surface, drilling fluid, well tools, and other drilling structures may be blown out of the wellbore. These “blowouts” often result in catastrophic destruction of the drilling equipment (including, for example, the drilling rig) and substantial injury or death of rig personnel.
Because of the risk of blowouts, blowout preventers (“BOP”) are typically installed at the surface or on the sea floor in deep water drilling arrangements to effectively seal a wellbore until active measures can be taken to control the kick. Blowout preventers may be activated so that kicks may be adequately controlled and circulated out of the system.
Just as a kick will propagate up the well, it may also enter the drill string and propagate through the inside of the drill string. To control a kick inside the drill string, a drill string internal blowout preventer (“IBOP”), sometimes called a “kelly valve” or a “kelly cock,” is used to seal off the drill string until measures can be taken to control the kick. An example of a Kelly cock is disclosed in U.S. Pat. No. 4,480,813, and incorporated herein by reference in its entirety. (An IBOP is sometimes called a “kelly valve” because, on older-style rigs, the IBOP was typically located near the “kelly,” which is a non-circular part of the drill string that is used to impart rotary motion to the drill string.)
An IBOP can be formed from of a variety of different types of valves, but a ball valve configuration is the most standard type. Ball valve type IBOPs typically include a valve ball that is located between two seats in the middle of a passage. The valve ball has a through hole, and can be rotated between two positions. U.S. Pat. No. 5,246,203 issued to McKnight et al. (“McKnight”), incorporated by reference in its entirety, discloses an oilfield valve that incorporates a ball valve mechanism. The mechanics of a basic ball valve mechanism are demonstrated in the McKnight patent. In a first position, as demonstrated in FIG. 1A, the through hole of the valve ball will align with the passage of the pipe or drill string. This position will allow complete, undisrupted fluid flow through the passage. The valve ball can then be rotated from this position into a second position, as demonstrated in FIG. 1B, to be misaligned with the passage of the pipe, disrupting fluid flow. Each of the seats surrounding the valve ball, one upper seat and one lower seat, seal against the valve ball, not allowing flow between the valve ball and the seat. Thus, the valve ball, coupled with the two seats sealing against the valve ball, can completely stop flow through the pipe passage when the valve ball is positioned to misalign with the through hole passage by having the seats seal up against the valve ball. In FIG. 1B, a seal is made between the seats, 101 and 102, and the ball valve, 105, to completely prohibit flow through the passage. The valve ball has the ability to seal against the seats to be effective against even the highest of pressures, allowing the arrangement to be used as an IBOP.
One issue with this type of ball valve arrangement is that when the valve ball is in the second position, blocking flow through the passage, as seen in FIG. 1B, a force is being transmitted to the valve ball across the diameter of the valve ball, placing the valve ball under a compressive force. The compressive force is translated through the ball to act at highest stress on points 110 and 111, where the points 110, 111 are relatively thin metal sections. The high stress may lead to deformation, crushing the ball. When the valve ball begins to deform, the generally round flow through passage of the ball warps into a narrower passage, representing that of an ellipse. After being deformed, when the valve ball is in the “closed” second position, the seal between the valve ball and the seats is undermined because of uneven contact between the seats and the valve ball. As well, in the “open” first position, the deformed valve ball will disrupt fluid flow through the passage because the elliptical through hole of the valve ball will not completely align with the round passage.
McKnight discloses an oilfield valve that includes a ball valve mechanism that seeks to reduce any valve ball deformation. An embodiment disclosed by McKnight is shown in FIG. 2, which shows an oilfield ball valve 200 that includes a valve ball 205, and an upper and a lower seat 210, 220 sealed against the valve ball 205. The lower seat 220 is supported by a spring 221 underneath, while the upper seat 210 is firmly positioned in the passage. When a fluid force is applied from above, the valve ball 205 translates the fluid force to the lower seat 220, and the lower seat 220 translates the fluid force to the spring 221 supporting it. If the fluid force is large enough, the spring 221 will compress. During compression, the lower seat 220 moves with the spring 221, and the valve ball 205 “floats” down to stay in sealing contact with the lower seat 220. Because the upper seat 210 is firmly positioned in the passage, the upper seat 210 does not move with the valve ball 205. The space between the upper seat 210 and the valve ball 205 provides an opening for the fluid to travel through. The fluid then fills up the valve ball 205, which translates the fluid force through the lower end of the valve ball 205 into the lower seat 220.
The translation of the fluid force is shown in FIG. 1B. As the fluid enters the valve ball 105, the fluid force moves from position 151, on the outer diameter on top of the valve ball 105, to position 152, on the inner diameter inside of the valve ball 105. In this fashion, the valve ball 105 will no longer experience a crushing load. Accordingly, the stress on the valve ball 105 is moved to section 115, one arcuate side of the valve ball. As a result, the fluid force is more directly translated to the lower seat 101.
An issue with having a ball valve arrangement with a spring loaded lower seat, as described above, is that the valve ball can only float in one direction. When fluid force comes from above the passage, translating down on the valve ball, the valve ball is able to float against the lower seat if necessary to prevent the valve ball from deforming. However, such an assembly does not allow the ball to float in the upward direction. Thus, when the valve ball is in its closed position and the fluid force coming from below the valve ball is too great, the valve ball will still deform from stress. The valve ball deformation is because the ball valve arrangement does not provide an opening for the upward flowing fluid, fluid flowing opposite to the direction the valve ball is allowed to float in, to travel into the valve ball and redistribute the stresses on the valve ball.
U.S. Pat. No. 6,662,886 issued to Russell (“Russell”) discloses another type of ball valve assembly and is incorporated by reference in its entirety. Russell discloses a ball valve that comes in a preassembled valve cartridge for use as a mudsaver valve. The cartridge ball valve assembly includes a sealing spring loaded lower seat, similar to that of the McKnight patent. A difference is that the ball valve disclosed by Russell does not have an upper seat that seals against fluid pressure exerted from below. Russell discloses that allowing pressure to be transmitted past the upper seat is advantageous because it allows for the pressure to be monitored by a pressure sensor upstream from the ball valve. For an IBOP, allowing pressure to transmit across the closed valve ball in either direction is undesirable.
For an IBOP, crushing amounts of fluid pressure may come from above and below. Accordingly, relief of the fluid pressure while preventing communication of the fluid pressure may be desirable in IBOP applications. What is still needed, therefore, are improved ball valve assemblies, particularly for use in IBOPs.